Historically, fingerprint image-capture devices have used optical-based sensors or capacitance-based sensors. With reference to the specific example of a fingerprint, optical sensors use a light source, lenses and a prism to image the “ridges” and valleys on the fingerprint, based on differences in the reflected light from the features. The conventional capacitance sensor uses semiconductor type processing to fabricate a two-dimensional array of capacitors. The individual sensors form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate. Upon contact with the array of sensors, the individual distance from each sensor to the skin is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint provide the means to replicate the fingerprint. An example of the use of capacitive sensors to measure the spacing is shown in FIGS. 1A and 1B.
Both the above techniques fundamentally measure the spacing between the fingerprint features, and the sensor. The measurement of spacing is inherently subject to several distortion effects: since the height difference between the ridges and valleys is only of the order of 50 microns, any parameter which affects the spacing between the finger and the sensor will affect the measurement. For example, both types of sensors are very sensitive to the thickness of the protective coating. They are also sensitive to oils or grease on the finger, and the presence or absence of moisture on the finger. In addition, most of these sensors are adversely affected by ambient temperature at the time of sensing, as well as electrostatic discharge (ESD). Under very hot or very cold conditions, the capacitive sensor can provide erroneous readings. ESD can altogether destroy a sensor. The combined effect of all these variables results in a very distorted image, if any, of the fingerprint, as shown in FIG. 1C.
As a result of the above drawbacks to spacing based reproduction of fingerprints, it would be very useful to be able to use the difference in pressure exerted by the ridges and valleys of a fingerprint on a sensor to replicate the fingerprint image. In principle, a pressure-based fingerprint sensor would be impervious to the drawbacks listed above, such as wet or dry conditions on the fingertip, presence of oil or grease on the fingertip, thickness of protective coatings, etc., providing a “digital” response, depending on whether the sensor experiences a ridge or not. This situation is illustrated in FIGS. 1D and 1E, where the pressure sensor can highlight only the ridges, which are the lines of interest in a fingerprint. However, due to a variety of factors, including the very low sensitivity and inability to provide the required resolution, pressure based sensors have not been deployed for the replication of fingerprints.
Improvements in pressure sensors are described in U.S. patent application Ser. No. 09/500,706 entitled “Magnetoresistive Semiconductor Pressure Sensor and Fingerprint/Verification Sensors Using the Same” and U.S. patent application Ser. No. 09/502,406 entitled “Use of Multi-Layer Thin Films as Stress Sensors, Appln., both assigned to the same assignee as the present invention. These applications describe many different improved aspects of pressure sensors, including sensors based upon magnetostriction and the GMR effect.
While the above applications provide many advantages, improvements that can result in greater sensitivity, less power consumption and lower thermal build-up are nonetheless desirable.
The present invention, as described below, described such a device, which, as one of it aspects, operates using the Tunneling MagnetoResistive (TMR) effect. The TMR effect was discovered relatively recently in the mid 1980's by Julliere et al. Since then researchers in the area of random access and flash semiconductor memories have explored the TMR principle to enhance the utility of such devices. This research, however, has been limited to these areas, and has not been reported as having usefulness in the context of fingerprint and pressure sensing applications.